This invention relates to power tire changing machines, and, more particularly, to an apparatus for controlling the stroke of a lower bead breaker shoe during the demounting operation of a tire from a wheel.
The majority of tires used on passenger cars and light trucks are of the tubeless type which are mounted on wheels having diameters varying from about 13 to 16 inches depending on the size of the vehicle. Most wheels generally have opposed outer rim flanges and upper and lower tire bead seats adjacent the rim flanges which lead to an inner, annular area usually of the drop center type. Many wheels in use today also include a raised annular safety hump or bead between at least one of the upper and lower bead seats and the annular drop center. Tubeless tires are formed with annular beads at the radially innermost portion of their sidewalls which are adapted to seat within the bead seats of the wheel creating an airtight seal therebetween. If the wheel is provided with one or more safety beads, the tire bead seats between the bead seat and safety bead to lessen the chance of the tire bead separating from the wheel during use.
Tire mounting and demounting machines have been in use several decades for the removal of tubeless tires from wheel rims. In demounting a tire from a wheel, the tire and wheel are first placed on the tire changing machine over a centerpost and then clamped in a horizontal position to the support table of the machine. The outer sidewall of the tire faces upwardly so that the valve core may be removed to enable the air pressure within the tire to drop to about atmospheric level. The upper and lower tire beads must then be broken loose from their respective bead seats adjacent the outer rim flanges of the wheel. Before the development of power tire changing machines, the bead breaking operation was accomplished by hand using chisel-like tools and hammers. Tire mounting and demounting machines now commercially available are power operated and adapted to displace both beads simultaneously or in successive order which greatly simplifies the demounting operation.
Most tire changing machines in commercial use employ an upper bead breaking shoe and a lower bead breaking shoe which are mounted on opposite sides of the tire and moveable toward one another. The upper and lower bead breaking shoes are forced against the tire beads on opposite sides of the tire to push the tire beads off of their seats and toward the inner drop center of the wheel.
In some power tire changing machines, the upper and lower bead breaking shoes move axially and generally parallel to the outer rim flanges of the wheel thereby moving the tire bead along the same path. It has been found, however, that mere axial movement of the tire beads often fails to remove them from their bead seats. If both the upper and lower shoes are advanced only in an axial direction parallel to the rim flanges, the tire is allowed to move radially outwardly from the wheel and both shoes tend to slide radially outwardly from the bead and push only on the sidewalls of the tire. Since the tire's sidewalls are relatively flexible, a force applied there tends to roll the bead on the bead seat area so that upon retraction of the shoes the beads both assume essentially their original position on the bead seats of the wheel.
It was recognized in prior art tire changing machines that the upper and lower bead breaker shoes must not only apply an axial force to the tire but they must also travel as closely as possible to the bead seat of the wheel to avoid sliding radially outwardly from the bead and pushing only on the sidewalls of the tire. One popular design in some tire changing machines was to adjust the radial inward stroke of the lower bead breaker shoe so that it engaged and followed the contour or profile of the wheel to insure contact with the tire bead was maintained. The problem with this approach is that the lower shoe often applied too much pressure to the wheel which frequently resulted in damage. This was particularly true in demounting aluminum and magnesium or "mag" wheels, and sport wheels, which were often scratched, cracked or severely bent. Problems also arose with a safety hump between the bead seat and annular drop center which is incorporated in some wheels to resist unseating of the tire bead during use. In bearing against and closely following the profile of the wheel, the bead breaker shoes of prior tire changing machines often hung up on the safety hump leading to damage of the rim and/or lower shoe. No provision was made in prior art tire changing machines to adjust the pressure applied to the wheel by the bead breaking shoe or control its radial inward movement once contact with the wheel was made.
To avoid damage to the wheels, and particularly mag and sport wheels, some prior art machines provided for the placement of the lower bead breaker shoe over the outer wheel flange prior to the demounting operation. Although this protected the rim flange, the stroke of the lower bead breaker shoe was not adjustable to prevent it from applying excessive pressure to the tire bead seat, safety hump or annular drop center. These portions of the wheel were still subject to damage.
Another attempt to protect wheels from damage was the provision in some prior art machines of a lock-out mechanism for the lower bead breaker shoe. The operator had an option to lock-out or prevent operation of the lower shoe and use the upper bead breaking shoe exclusively. The problem with this approach is that the tire and wheel first have to be mounted on the tire changing machine, the upper bead broken and then the tire turned over to break the lower bead in a separate operation. This is a time consuming and inefficient method of mounting and demounting a tire.